An amplifier typically has low efficiency and large linearity margins at low power regions, and high efficiency and small linearity margins at high power regions. For linear amplifiers, the linearity is limited at the highest output power condition, which is known as the saturated region. The linearity and efficiency of an amplifier may be affected by the bias conditions of the amplifier. Adaptive biasing generates an appropriate bias to enhance the performance of power amplifiers with respect to their input and/or output power levels.
Amplifiers may be classified depending on their associated bias level and current conduction angle. These classifications include class-A, class-B, class-AB, and class-C amplifiers. For instance, a class-A amplifier has the highest bias level with the highest linearity, and class-C has the lowest bias level with the lowest linearity. In contrast, class-A amplifiers have the lowest efficiency, and class-C amplifiers have the highest efficiency. This is because typically the efficiency of an amplifier has than opposite reaction to bias conditions than that of an amplifier's linearity.
However, if the bias of the amplifier is controlled adaptively, it can achieve better performance compared to an amplifier with fixed bias conditions. For instance, if the amplifier is biased near class-B at a low power region and class-A at a high power region, it can achieve better efficiency at the low power region and better linearity at the high power region while still meeting acceptable linearity specifications at the low power region and acceptable efficiency specifications at the high power region.
Fundamental configurations of most conventional adaptive biasing schemes for power amplifiers are composed of a power detector component, a low-pass filter, and a bias voltage or current generating component. FIG. 1 shows a schematic diagram for a conventional power amplifier with a conventional adaptive bias circuit providing a feedback signal. A power amplifier with a feedback signal has a highly efficient linear performance. For the power amplifier 12 shown in FIG. 1, an output signal is sampled by an output sampler 14, and the sampled signal is then filtered by a filter 16. The filtered signal power is detected by a detector 18, and the detected signal is controlled by a control circuit 20. This controlled signal re-biases the power amplifier 12. The control circuit 20 adjusts the power amplifier 12 to maximize efficiency with an allowable distortion.
Another schematic diagram for a power amplifier with a conventional adaptive bias circuit is shown in FIG. 2. FIG. 2 shows a class C amplifier that has linearized with dynamic biasing. The bias circuit includes a sampling stage, two current mirrors with RF filters, and a resistive divider to provide dynamic biasing. This configuration shown in FIG. 2 makes a class C amplifier work like a class B amplifier, but with increased efficiency.